The "Best?" That would be the Four Seasons (located on the top floors of Mandalay Bay).
The Venetian is pretty overwhelming (I don't like it, I won't even gamble there). Too overdone, IMHO.
Bellagio is expensive, but a very nice house. Design and appearance is very nice.
The Mirage is a nice place, very tropical, nice rooms, good food, good location.
If you're younger, the Hard Rock Hotel is pretty popular ... not known for "serious romance," but it has some interesting spots. It is off-strip (WAY off-strip) but you could almost walk to the airport.
I still prefer the Rio. All rooms are suites (basically just a very large room), something like 16 restaraunts in-house (ranging from five star to buffets, all that I've tried have been excellent), the hottest club in Vegas (now one of ~three "hot spots" at the top), nice casino, etc. They have ~three sand-beach pools, a Spa, one of the best golf courses in the area (located off-property), a cocktail lounge on the top floor (53 floors?) with an open terrace (the Voodoo Lounge).
It is located off the strip (~ less than a mile to Flamingo and LV Blvd), but there are free shuttles to Harrah's (in the middle of the strip), and more cabs than you can shake a stick at. "Off the Strip" can be an advantage or disadvantage, depending on your inclination. I prefer off-strip because it reduces the tourist count.
If you plan on renting a car, go for it: all hotels / casinos are free valet parking (don't forget to tip). I don't recall ever seeing any kind of paid parking in Vegas.
Good Luck
Scott